1. Field of the Invention
The present invention relates generally to the production of electric power by use of geothermal fluids and more particularly to processes for reducing the heavy metal content of geothermal sludge formed during use of geothermal brine to produce electric power.
2. Discussion of the Prior Art
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of energy potential, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole or geyser activity. Thus, as an example, geothermal aquifers are fairly common along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal steam or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has, instead been directed to exploitation of geothermal resources for production of electrical power which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of petroleum products used for conventional production of electric power, as well as actual or threatened petroleum fuel shortages or embargos have intensified the interest in use of geothermal fluids as an alternative and generally self-renewing source of power plant "fuel".
General processes by which geothermal fluids can be used to generate electric power are known and have been known for some time. As an example, geothermal steam, after removal of particulate matter and polluting gases such as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation can typically be reinjected to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in binary systems in which a low-boiling point, secondary liquid is vaporized by the hot geothermal liquid, the vapor produced being used to operate gas turbine generators.
As might be expected, use of geothermal steam is preferred over use of geothermal water or brine for generating electric power because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, by the late 1970's geothermal steam at The Geysers in Northern California was generating about two percent of all of California's electricity consumption.
While energy production facilities at important geothermal steam sources, such as at The Geysers, are still being expanded, when not already at capacity, the known number of important geothermal steam aquifers is small compared to those of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water than it is for geothermal steam. As a result, considerable current geothermal research is understandably directed towards the development of economical geothermal brine and water electric generating plants, much of this effort being expended towards use of vast geothermal brine resources in the Imperial Valley of southern California.
Although, as above mentioned, general processes are known for using geothermal brine or water for production of electric power, serious problems, especially with the use of highly saline geothermal brine, have often been encountered in practice. These problems have frequently been so great as to prevent the production of electric power at competitive rates and, as a consequence, have greatly impeded the progress of geothermal brine power plant development in many areas.
These severe problems relate primarily to the typically complex composition of geothermal brines. At natural aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of 400 to 500 psig, the brine leaches large amounts of salts, minerals and elements from the aquifer formation. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica as well as substantial levels of dissolved heavy metals such as lead, zinc and cadmium. In addition, many other impurities, particulate matter and dissolved gases are present in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam conversion (flashing) stages, silica saturation levels in the brine are typically exceeded and silica precipitates from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast growing rates, extensive facility down time for descaling operations may commonly be required at some geothermal brine facilities. Injection wells may also require frequent and extensive rework and new injection wells may, from time to time, have to be drilled at great cost.
Considerable effort has, as a consequence, been directed towards developing processes for eliminating or controlling silica scaling from geothermal brine. One scale reduction process of particular interest involves induced precipitation of silica from the brine in the flashing stage by utilizing seed crystallization techniques. Thus, when silica saturation levels are exceeded in the flashing vessels, the "excess" silica preferentially crystallizes from the brine onto seed crystals which are intentionally introduced into the vessels. Typically, the crystallized silica precipitate is settled from the brine in a downstream reactor-clarifier stage, the clarified brine being flowed on to a filtering stage and then to a reinjection stage. Some of the silica precipitate (sludge) from the reactor clarifier may be pumped back upstream into the flash crystallization stage as seed material, the remainder being dewatered and removed from the facility for disposal. The amount of such silica sludge requiring disposal is relatively large; for example, for a 10 megawatt power plant requiring a brine flow rate of about 1.3 million pounds an hour, as much as about six tons a day of silica sludge may be produced and require disposal.
During the silica crystallization process, many other materials are removed from the brine along with the silica. Thus, the produced sludge, herein referred to as silica sludge, although mostly silica, may also contain significant amounts of barite and heavy metals, such as lead, copper and zinc, which above specific levels of concentration, may be considered as toxic and therefore require disposal at specially designated toxic waste dumps. The costs associated with disposal of toxic silica sludge are substantial and can be expected to increase as additional and larger geothermal brine power plants are constructed and produce more sludge, as allowable concentrations of heavy metals in the sludge are reduced to meet anticipated stricter environmental requirements and as toxic waste dumps become fewer and/or more remotely located.
As an alternative to disposing of silica sludge as a toxic waste, attempts have been made to detoxify geothermal brine sludge by substantially reducing the concentration therein of heavy metals, particularly of lead, copper and zinc, below allowed levels. However, because of the complex composition of the geothermal brine, and hence of the silica sludge crystallized and precipitated therefrom, conventional heavy metal extraction processes have been found not to work as expected and/or not to be economical compared to the cost of transporting the untreated sludge to a toxic waste disposal site.
Therefore, an object of the present invention is to provide a relatively effective and economical process for reducing the heavy metal concentration of geothermal brine sludge.
It is another object of the present invention to provide a process for reducing the concentration of heavy metals in geothermal brine sludge which is compatible with present types of geothermal brine power plants.
Additional objects, features and advantages of the present invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawings.